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Discharge lamp lighting apparatus and discharge lamp lighting methodDischarge lamp lighting apparatus and discharge lamp lighting method description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060197473, Discharge lamp lighting apparatus and discharge lamp lighting method. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This application claims the priority benefit under 35 U.S.C. .sctn.119 of Japanese Patent Application No. 2005-060777 filed on Mar. 4, 2005, which is hereby incorporated in its entirety by reference. BACKGROUND OF THE INVENTION [0002] 1. Field [0003] The disclosed subject matter relates to technology for an apparatus and a method for lighting a discharge lamp including a vehicle headlamp with improved start-up variations in luminous flux. [0004] 2. Description of the Related Art [0005] FIG. 1 is a diagram showing an example of circuit configuration of a conventional discharge lamp lighting apparatus (for example, see Japanese Patent Laid-Open Publication No. 2003-338393). This conventional discharge lamp lighting apparatus (hereinafter, also referred to simply as lighting apparatus) exemplifies the case where the discharge lamp is used as a vehicle headlamp, and is started with a starting current several times as high as the rated current for the sake of instantaneous lighting. [0006] The conventional lighting apparatus has a DC/DC converter 2, a DC/AC inverter 3, and a starting transformer 4. The DC/DC converter 2 boosts a direct-current voltage input from a direct-current power supply 1. The DC/AC inverter 3 converts the boosted input from direct-current to alternating-current. The starting transformer 4 generates a high-voltage starting pulse for making a discharge lamp 5 start to discharge at the time of lighting. A lamp power control circuit 8 controls the boosting of the DC/DC converter 2 in accordance with values detected by a lamp voltage detection circuit 6 and a lamp current detection circuit 7. [0007] Note that the circuits described above are intended for alternating-current lighting. To light the discharge lamp 5 with a direct current, the DC/AC inverter 3 is omitted from the circuit configuration. [0008] By the way, the direct-current boosting circuit (DC/DC converter 2) contains a lighting control circuit which controls the lamp current or lamp power according to the lamp voltage from the start to stabilization of the discharge lamp 5. [0009] More specifically, the lighting control circuit determines the set value of the lamp power through calculation. The lighting control circuit then controls the lamp power of the direct-current boosting circuit based on a supply output control method such as pulse width modulation (PWM). As above, the lighting control circuit of the discharge lamp lighting apparatus intended for automobile illumination is provided with a PWM or other control circuit that detects the lamp voltage and performs feedback control so that the lamp power has the predetermined set value. Then, in this discharge lamp lighting apparatus, the control circuit for lighting the discharge lamp instantaneously detects the lamp voltage and the lamp current and performs feedback control based on those values, thereby controlling the lamp power from immediately after start-up to stabilization. [0010] The reason why such a control as described above is performed here will be described below. Ordinary discharge lamps including a mercury lamp take a long time to reach sufficient brightness. To brighten the lamps earlier, the lamp currents can be increased for quick start-up at the initial phase of lighting. Subsequently, the lamp currents at the initial phase of lighting should be reduced to the rated currents as the discharge lamps increase in brightness. Here, at the initial phase of lighting, the discharge lamps have the characteristic that the lamp voltages immediately after cold start are as low as 20 V or so, and the lamp flux as low as 10% to 20% or so as compared to during stable time operation. [0011] In general, discharge lamps experience a rise in lamp voltage and lamp flux as their lamp tube temperatures increase. In order to achieve a required intensity of illumination quickly, discharge lamps are subjected to so-called warm-up currents that are several times as high as their rated currents while the lamp voltages are still low immediately after start-up. [0012] Due to these warm-up currents, the discharge lamps quickly increase in tube temperature. The discharge lamps experience a sharp rise in the values of both the lamp voltage and the lamp flux accordingly. At this rise of luminous flux, the lamp voltage and the lamp flux show almost the same tendencies upward with a minor difference in time. That is, when the lamp voltage is low, the flux is also in a low state. When a maximum warm-up current is passed, the lamp voltage rises and the lamp flux gradually rises as well. Then, the lighting control circuit performs a control operation to reduce the lamp current that is higher than the rated current at the time of warm-up, into the stable rated value in accordance with the rise in the values of the lamp voltage and lamp flux (lamp current reduction control). [0013] Now, in view of environmental concerns in recent years, so-called mercury-free HID (High Intensity Discharge) lamps, which are capable of lighting without containing mercury, have been under development. [0014] Now, description will be given of differences between the lighting controls on conventional HID lamps and mercury-free HID lamps. [0015] FIG. 2 is a diagram showing an example of conventional power control on a mercury-containing HID lamp. In conventional HID lamps, the lamp voltage immediately after start-up is 20 V or so while the lamp voltage rises up to around 85 V at stable time. The lamp voltage characteristics immediately after start-up and at stable time thus have a difference in value. In conventional HID lamps, it is therefore easily possible to determine the state of the lamp voltage, whether it is a value corresponding to a time immediately after the start of lighting or at a stable time, even if some range of variations in the actual lamp voltage is taken into account. It has thus been possible to control the lamp current appropriately according to the state. [0016] In contrast, mercury-free HID lamps show a lamp voltage of 25 V or so immediately after start-up, and as low as 42 V even at stable time. The voltage difference therebetween is thus smaller than in the conventional HID lamps described above. Besides, in mercury-free HID lamps, the range of variations of the lamp voltage at stable time is as large as .+-.20% or so with respect to the rated lamp voltage. Thus, for mercury-free HID lamps it is hard to determine which state they are in, immediately after start-up or at stable time, based on the difference in the lamp voltage. [0017] Consequently, it has been difficult to control the lamp current according to the situation, and by extension to perform an appropriate warm-up control on the flux start-up, when mercury-free HID lamps are subjected to the conventional power control. [0018] By the way, while the warm-up current is applied to a discharge lamp as described above, the control circuit detects the lamp voltage in order to perform power control from the starting-time power to the stable-time power. Typically, this lamp voltage is not directly detected from across the lamp to which a high-voltage pulse of 20 kV or so is applied at starting time, but detected so as to include the starting transformer for generating the high-voltage pulse, which is connected in series to the discharge lamp. The reason for this is that it is difficult to detect a voltage from the point where the starting pulse is applied. Here, the inductance of the starting transformer may cause some errors. Depending on the direct-current bias characteristic, however, impedance errors caused by the inductance have only a minor effect since the discharge lamp of instantaneous start type is typically lit by a rectangular wave having a low frequency of several hundreds Hz. [0019] Nevertheless, the mercury-free HID lamp shows a resistance of 3 to 5.OMEGA., combining a winding resistance resulting from the warm-up current at starting time and an ON resistance of the bridged inverter FET in the prior stage (DC/AC inverter 3). Then, the mercury-free HID lamp requires a lamp voltage of 25 V or so immediately after start-up, when a maximum starting current of 3 A or above must also be applied. In consideration of the foregoing, the above resistances produce errors of 9 to 15 V in addition to the lamp voltage. Under these circumstances, it has been difficult to detect the lamp voltage accurately during the warm-up control of the mercury-free HID lamp. This makes the starting-time control on the mercury-free HID lamp difficult to optimize appropriately by means of the lamp voltage. This is also shown from the fact that the current increases and the errors increase as the lamp voltage decreases. The mercury-free HID lamp has a stable-time lamp voltage as low as 42 V, or approximately half that of the mercury-containing HID lamp. [0020] It has thus been difficult to determine which state the mercury-free HID lamp is in, starting time or stable time, because of the foregoing characteristics as well as the fact that the detection values of the lamp voltage in the two states have little difference if the resistor-based errors mentioned above are included in the detection values of the lamp voltage. It has thus been extremely difficult to perform power control on the mercury-free HID lamp based on the lamp voltage at the time of starting the lamp. [0021] FIG. 3 is a chart showing an example of the voltage detected for mercury-free HID lamps. In FIG. 3, the lamp voltage is around 25 V immediately after start-up, with variations of around .+-.5 V lamp by lamp. From this FIG. 3, it can be seen that it is extremely difficult to perform appropriate start-up control when the starting power is controlled in accordance with the lamp voltage as mentioned above, due partly to detection errors of the lamp voltage. [0022] FIGS. 4A to 4C are diagrams showing examples of conventional power control on a mercury-free HID lamp. FIGS. 4A, 4B, and 4C show the cases where the lamp voltage is low, average, and high, respectively. [0023] In general, when a mercury-free HID lamp is subjected to start-up control according to the lamp voltage, the lamp voltage itself has a range of variations of 10 V or so immediately after the start of a discharge. As indicated above, the stable-time lamp voltage can be as low as 42 V. Suppose that the conventional start-up control is applied to the mercury-free HID lamp without considering the above-noted variations. For example, the lamp of FIG. 4A, having a lamp voltage of 20 V immediately after start-up, undergoes a maximum current for a relatively long period with respect to the lamp of FIG. 4B, having a lamp voltage of 25 V immediately after start-up as a reference. On the other hand, the lamp of FIG. 4C, having a lamp voltage of 30 V immediately after start-up, undergoes a maximum current for a relatively short period. If the start-up control is thus performed on the mercury-free HID lamp without considering the variations, large variations in the flux start-up characteristic can occur. The mercury-free HID lamp thus has had the problem that the variations, if any, can cause both excessive warm-up with the result of luminous flux overshoot, and insufficient warm-up with the result of slow start-up. Continue reading about Discharge lamp lighting apparatus and discharge lamp lighting method... 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